1. Field of the Invention
The present invention generally relates to a control circuit of a charge pump circuit, and more particularly, to a control circuit considering both the efficiency and the driving ability of the charge pump.
2. Description of Related Art
In an electronic apparatus, it is often to require a plurality of source voltages with various voltage levels, so that a charge pump circuit is often disposed to use an available source voltage to produce the source voltages with various voltage levels.
FIG. 1 is a block diagram of a conventional charge pump circuit and the control circuit thereof. The control circuit 100 herein includes a voltage level detector 102 and a ring oscillator 104. The voltage level detector 102 is for detecting the voltage level of the output voltage Vout of the charge pump circuit 106. When the output voltage Vout is less than a constant predetermined voltage level, the voltage level detector 102 outputs a triggering signal S1 to the ring oscillator 104, and the ring oscillator 104 outputs the clock signal P1 according to the triggering signal S1, so that the charge pump circuit 106 pulls the output voltage Vout back to the normal level according to the clock signal P1. In general speaking, when the frequency of the clock signal P1 output by the ring oscillator 104 is high, the driving capacity of the charge pump circuit 106 is better but has a lower efficiency. In contrast, when the frequency of the clock signal P1 output by the ring oscillator 104 is low, the driving capacity of the charge pump circuit 106 is worse but has a higher efficiency. Since the frequency output by the ring oscillator of the conventional charge pump circuit is fixed, so that the circuit design must face a trade off between the driving ability and efficiency of the charge pump circuit according to the application practice; it is not the best of both worlds.
U.S. Patent Application 20060197583 discloses a method to increase the efficiency of a charge pump circuit, where the input frequency of the charge pump is determined by the load amount driven by the charge pump so as to advance the efficiency of the charge pump circuit. The method uses a constant voltage level to decide the frequency conversion of a ring oscillator, so that the scheme of the patent application still fails to obtain a compromise solution between the efficiency of charge pump circuit and the driving capacity. It should be noted that when the output voltage drifts back and fourth about the predetermined voltage level, the ring oscillator would keep switching the frequency so as to low down the efficiency of the charge pump circuit; even the ring oscillator fails to produce the clock signal required during switching the frequency for the charge pump circuit to increase the output voltage. As a result, the output voltage Vout drops down.